Monday, May 18, 2009

Kitt Peak and Infrared Fun!

I've been having fun with my modified digital Canon 20D - the normal infrared blocking filter was replaced with a infrared passing filter giving some new effects when looking at normal objects. I was looking forward to our Saturday trip to Kitt Peak National Observatory to try some new perspectives. The "Wood's Effect" of IR photography makes live foliage look white, and the blue sky and bodies of water dark. I also know that the telescope domes use titanium dioxide paint to keep out as much heat as possible, so would also appear bright white.

Back in olden days when I worked at the Observatory, there were lots of scientific photographic emulsions available, IR-sensitive as well. I had worked with IR before, and used my own cameras as well as some interesting cameras Kitt Peak had to take shots similar to these 25 years ago. One of the more interesting ones was a big box camera made by one of the solar astronomers that had an 800mm focal length lens and used 8"X10" glass plates. Those were some amazing images. I'll have to dig out some of those plates and scan them - with a deep red filter, and a 30 second exposure the resolution was exquisite and haze penetration was astounding. So I was interested in how close I could get to those results with today's digital detectors.

Here are a couple comparisons with the same lens set to the same zoom setting taken a few minutes apart on a hazy afternoon - the IR image with the modified 20D and the color images with the Canon Xsi. As always, click on the image to load a full-size version. The main thing to note is the dark sky - higher contrast of any cloud features, white vegetation, and good haze penetration. Note that there is a little subtle color - the sky sometimes comes out a little sepia toned, and vegetation a little blue. As I understand it, normally only the red-filtered pixels should show any IR signal, but the blue and green-filtered ones have a varying amount of IR leakage, so some subtle colors result. This effect can be magnified in Photoshop, but these are basic out-of-the camera results, using the custom white balance the camera-modifier set for me. I'll get a little more adventuresome later!

Another experiment before me is to use a deeper IR filter. The filter Jim Chen installed in front of the detector (in all operational respects, the camera behaves like a normal camera with the filter in front of the sensor) cuts on right at the limit of the eye's sensitivity. I've got a filter or two that transmit much further into the IR, so should get much better haze penetration at the expense of not seeing anything through the viewfinder, since this other filter will be in front of the lens! There are also some interesting astronomical applications - for sure I'll be bringing the camera to the RTMC telescope makers conference this weekend - hopefully Olivier Thizy will be there with his spectrograph again and I can directly measure the spectral sensitivity! I foresee fun times ahead!

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